Chemical-mechanical planarization (CMP) is a well known process used in semiconductor fabrication to polish a surface, for instance a dielectric film surface formed on a silicon wafer or other workpiece. The CMP process removes small elevated features on the surface without significantly thinning the film on, for instance, the flat areas lower than the elevated features.
As shown in a cross-sectional front view (FIG. 1), a typical CMP system uses a flat, rotating disk ("platen" 112) with a pliable polishing pad 110 mounted on its upper surface 112S. As platen 112 is rotated, a slurry (not shown) is deposited near the center of the pad's surface 110S and spread outward using, at least in part, centrifugal force caused by the rotation. A wafer 106 (or substrate), held by a carrier 102 positioned above pad 110, is then pressed downward against pad's surface 110S such that the rotating polishing pad 110 moves the slurry over the wafer's surface 106S. In this manner, elevated spots of the wafer surface 106S are removed and a substantially planar surface is achieved. It is to be noted that although pad surface 110S is illustrated as a smooth surface for simplicity of illustration, in fact pad 110 has a rather roughly textured surface 110S to be rubbed against wafer surface 106S along with the acidic (or basic) slurry containing abrasives.
Pad 110 is made of, e.g., polyurethane impregnated felt, cast and sliced polyurethane with filler, cast and sliced polyurethane without filler, or composite of two or more types of pad material. For effective polishing, the pad's surface 110S needs to have a flat or bell-shaped profile. After a certain period of CMP polishing, the profile of the pad's surface 110S is altered to be no longer useful for polishing, resulting in inefficient and sometimes even ineffective polishing. Deterioration of the profile happens as the pad surface loses its rough texture through mesa formation. Therefore, to maintain the quality of planarization, pad's surface 110S needs to be rejuvenated (i.e., a desirable profile of surface 110S restored) once in a while by, for example, raising the naps of surface 110S by scraping surface 110S.
One problem with prior art pad rejuvenation is that it has to be done "off-line" and ex situ, i.e., either before or after the polishing event with the wafer removed from the polishing station, reducing the throughput of the polishing process. Another problem with prior art pad rejuvenation (which is performed ex situ) is that a uniform removal rate is not maintained, since areas of pad surface 110S are conditioned only in the beginning of the polishing process and, as areas of pad surface 110S are used one or more times for polishing, the removal rates of those areas gradually drop.